Recirculated gas damper control method and system for vapor generating and superheating units



Aprll 12, 1960 A. R. HARMAN ET AL 2,932,288 RECIRCULATED GAS DAMPER CONTROL METHOD AND SYSTEM FOR VAPOR GENERATING AND SUPERHEATING uurrs Filed Aug. 5, 1954 4 Sheets-Sheet 1 INVENTOR5 $404 foe/1 BY ATTORNEY April 12, 1960 A. R. HARMAN ETAL 2,932,288

RECIRCULATED GAS DAMPER CONTROL METHOD AND SYSTEM FOR VAPOR GENERATING AND SUPER-HEATING UNITS Filed Aug. 5, 1954 4 Sheets-Sheet 2 A54 /5/- INVENTORS ATTORNEY April 12, 1960 A. R. HARMAN ET AL 2,932,238

RECIRCULATED GAS DAMPER CONTRQL METHOD AND SYSTEM FOR VAPOR GENERATING AND SUPERHEATING UNITS gap [94 /62 i 204 Z06 [Mr/r50 ji /40721014 77C EXA/Az Jf i -em EMF/IMAM 5 1 7250/44 SW/mv Jazz/vow INVENTORS APA/A/JTJA/IQ/I/AFMAA/ BY B4114 AKA ac ATTORNEY 1 1960 A. R. HARMAN ETAL 2, 32,

RECIRCULATED GAS DAMPER CONTROL METHOD AND SYSTEM FOR VAPOR GENERATING AND SUPERHEATING UNITS Filed Aug. 5, 1954 4 Sheets-Sheet 4 B Paul H Jfbciz I i imA-nomav RECIRCULATED GAS DAMPER CONTROL METH OD AND SYSTEM FOR VAPOR GENERATING AND SUPERHEATING UNITS Application August 5, 1954, Serial No. 448,062

13 Claims. (Cl. 122-478) This invention relates to apparatus and a method for the control of gas flow and gas temperature conditions in a furnace over a gas path which may be included in a vapor generating and superheating unit.

One aspect of the invention is exemplified in a vapor.

generating and superheating unit which includes a furnace, steam generating tubes subject to the heat of the furnace, and a superheater having tubes which are subjectrto the heat from the gases originating in the furnace. A pertinent vapor generating and superheating unit involves a gas recirculationsystem including a recirculating gas fan and ductwork withdrawing the heating gases from a position downstream of the superheater and introducing these gases into the furnace or' at a position in the gas flow path ahead of the superheaten' When the flow of recirculated gas through this system is variably controlled so that the flow of recirculated gas increases as the rate of vapor generation is decreased an inherent tendency of a convection superheater will be compensated in order that a predetermined, final steam temperature may be maintained over a wide load range. With such a vapor, generating and superheating unit, and with changing rates of furnace firing and changing rates of recirculated gas flow there occurs a condition under which the pressure of the unrecirculated gases is greater than the pressure of the recirculated gases at the outlet of the recirculated gas system and when this condition occurs, there may be reverse. flow of the high temperature of recirculated gases into the recirculated gas system, and such a reverse gas flow may cause substantial damage to the recirculated gas fan. This invention involves means and such a method of operation that such a reverse flow of the recirculated gases is automatically prevented in' the event of the undesirable pressure conditions mena tioned above In obtaining the above indicated result, the-invention, in one of its broadest aspects, presents a heat exchange unit which includes a zone through which high tempera-.

ture gases normally flow. Communicating with this zone is the outlet passage of a fan having its inlet connected to a source of lower temperature gases. Within this outlet passage there is a gas flow regulator exemplified by a tight shut-off damper, and associated with this damper is a damper control and operating means governed 'by a differential pressure control means in communication with the in-fiow and out-flow sides of the damper. This means measures the'diiference in pressure at these points and when that pressure difierence' reaches a predetermined value below which there could be reverse gas flow toward the fan, the damper is moved to its completely closed condition. For example, in the illustrative vapor generating and superheating unit the damper for controlling the flow of lower temperature gases into the zone of the higher temperature gases is associated with a differential pressure switch which is set to close the damper completely when the difference in pressure on the two sides of the damper is 'below 1" Of water. The damper and the load on the recirculating gas fan is' nor- Ice mally subject to control from a superhe'at temperature controller and it is only when this controller calls fora small or near minimum flow of recirculating gas that the differential pressure switch is operative. Whenthe load on the recirculated gas fan, as represented by the. current delivered to the motor for operating the fan, is belowl rated motor current load the pressure differential switch control of the damper is operative as above-described, but at fanloa'ds above /2 load there is another connection between the differential pressure switch control and a motor current control to render the differential pressure switch inoperative.

The 'diiierential pressure switch control and the motor current control are so associated with the damper operation that when the motor current'is below V2 of its rated current and when the difierential pressure across the damper is less than 1" of water, the damper is closed.

The invention also involves the simultaneous introduction into the passage leading from the recirculated gas fan to the zone of high temperature gases, of cooling air to prevent temperature damage to the fan or its associated elements.

The invention is applicable to a vapor generating and superheating unit having a recirculated gas system which may be used in either or both of two manners. One manner of use of recirculated gas involves such introduction of the recirculated gas into the flow of unrecirculated gases that steam temperatures may be controlled both by the elfect of the lower temperature gases in decreasing therate of heat absorption in the vapor generating tubes and increasing the heatavailable for superheating, the generatedvapor as the rate of vapor generation decreases. The other manner of introduction of recirculated gases is forthe purpose of tempering the heating gases, or, for reducing the temperature of the gases to an allowable value before they contact the superheated.- The latter method of operation .would involve the recirculation. of a maximum amountof the lower temperature gases at high rates of vapor generation, with the rate of gas recirculation decreasing as the rate of vapor generation decreases The invention is applicable to either use of recirculated gases p -Ce rtain features of the invention are disclosed in the co-pending application of Paul H. Koch, S.N. 167,073, filed June 9, 1950, now US. Patent 2,737,931.

The invention will be concisely set forth in the claims appended'hereto, but for a more complete understanding of the invention, its uses and advantages, recourse should behad to the following description which refers to drawings illustrating the invention:

: In the drawings: 7

Fig. 1 is a somewhat diagrammatic sectional elevation of a vapor generating and superheatin-g unit involved, in the invention;

a Fig. 2 is a schematic air-gas flow diagram indicating some of the components of the invention, and their relative arrangement; i

;Fig. 3 is a control diagram;

Fig. 4 is a diagrammatic layout of'a control system applicable to a reduction to practice of the invention;

Fig. 5 is a diagram of a modification of the Fig/4 control system.

The vapor generating and, superheating unit of Fig. 1

involves a large volume vertically elongated furnace the wa a inwardly along the underside of the arch 26, and from the nose of this arch some of these tubes extend rearwardly as indicated at 28, along the bottom of the superheated gas pass. Thence they extend in screen formation f across the flow of-gass from the superheater gas pass," as indicated at 30.' They continue along the roof pf the unit to the drum 20. Others of the tubes 22 extend upwardly from the nose of the arch 26 to form the screen. 32 in front of the secondary superheater 34. From the top of the superheater gas pass these tubes extend along the roof 36 to the drum '20. At the lower part bf the unit the rear tubes 22 are inclined downwardly indicated at 38 and the lower ends of'the tube 16 are' oppositely inclined as indicated at 40 to form the hopper bottom for the furnace.

The lower headers 18 and 24 are appropriately connected into the fluid circulation system of the unit by :suitable downcomers and circulators and there is a simiflararrangement of wall tubes and circulatory connections for the remaining walls of the furnace.

j The furnace is fired byappropriate burners positioned at 42'as in position along the wall 12.

The furnace gases after leaving the outlet of the gas pass for the secondary superheater 34 pass into the tuming space 44 at the top of a downfiow gas pass having the rear wall 45. In this gas pass the gases pass over the banks of spaced tubes 46-49 of the primary superheater. This superheater has an inletheader 50 which receives, vapor from the drum 20 through the tubes 52 f extending along the top of the unit to the header 54. From this header the vapor flows downwardly through the tubes 56and along the roof of the downflow gas pass and then along the wall 45 to the header 50.

" Beyond the primary superheater the gases pass over a 7 bank of tubes 60 constituting the economizer. Beyond the economizer the gases pass through the duct 62 which is connected to stack, with a side connection to the inlet duct 64 of the recirculated gas system. This system includes the duct 66 leading to the inlet of the recir- 'culating gas fan 68, the outlet of which is connected by ductwo'rk' to openings between the lower parts of tubes 22, at the throat 72 of the hopper bottom of the furnace.

-' The Fig. 1 unit has a forceddraft fan 74 the outlet of which is connected by ductwork 76 to the air heater 78. From the air outlet of the air heater the heated air passes through the ductwork components *8 2 to the -windbox 84 from which the heated secondary air passes 7 gas to the furnace.

through theregisters of the burners 42. Leading downwardly and laterally from the duct 82 is the bypass duct 86, dampered at 88, and connected to theductwork 70 for the recirculated gas system. Fig. 2 is intended to indicate diagrammatically an airgas flow system involvinga recirculated gas system as applied to a vapor generating and super-heating unit which corresponds generally to the unit shown in Fig. l. The recirculated gas fan 68' has its inlet connected by a duct 66 to the gas outlet of a fluid heating unit which is intended to represent a unit inclusive of such elements as the furnace 10- of Fig. 1, its connected gas passes and the superheaters disposed therein. 7 Recirculated gases from the outlet of the fan flow through the duct 70' to a position 72' within the fluid heating unit 90, the position 72 corresponding generally to the position of re circulated gas entry 72, of the Fig. 1 unit.

A forced draft fan 74', corresponding generally to the forced draft fan 74 of Fig. l delivers air through the duct 76' to the airheater 78"from which the heated air flows throughthe duct 82' to the burner windbox 84'. The flow of air from the forced'draft fan 74' through the duct 76' is regulated by the damper 92, and heated air flowing from the airheaters 78' to the burners is sub ject to control by the dampers 94 and 96, the latter being considered here as the air register control associated burners is also subject to the operative influence of the by-pass $8 communicating with the duct 82' at a position below the airheater 78 and leading to the inlet duct which communicates with the inlet of the forced draft fan 74. Control of air flow through this bypass circuit is subject to the positioning of the dampers 102 and 104.

From the position along the duct 76 the bypass duct 86 leads to the outlet duct 70' of the gas recycling system, the flow through this bypass being subject to control by the damper'88.

In the generating and superheating unit of the Fig. 2 diagram, heating gases pass over the airheater 78 and y then through the upper part of the unit 108. Thence they pass through a stack to the atmosphere.

The damper 112 in the duct 66' is subject to a plurality of controls. One control involves the differential pressure switch 114 having its opposite sides connected by ducts or tubular elements 116 and 118 to the duct 66"at inlet and outlet sides of the damper 112. This control is automatically operative to tightly shut off the damper 112 and stop allfiow of recirculated gas through the fan 68f when the differential between the pressure at the inlet of the tubular element 116 and the pressure at the inlet of the tubular element 118 is less than a predetermined value, such as 1" of water. Another control which is effective upon the differential pressure switch control and therefore'effective upon the operation of the damper 112 is a motor current control responsive to the current flowing through the electric motor for driving the fan 68 and this control is preferably operative at fan loads above one half full rated current load to 'render the differential pressure switch control inoperative.

Below one half load the differential pressure switch is operative. This motor current control superimposed upon the difierential pressure switch control will be described below. A third control for the damper 112, and therefore for the control of the recirculated gas flow into the furnace of the Fig. 1 unit or the heat exchange unit 90 is indicated in Fig. 4. This is a superheat or boiler pontrolwhich operates to control the flow of recirculated It is responsive to changes in steam or vapor flow and changes in the final steam or vapor temperature at the outlet of the secondary superheater and to atactor involving the product of gas mass flow across thesuperheater and the temperature of the gases. These operative influences act conjointly to vary and control the recirculated gas flow to the end that final steam temperature may be maintained at a predetermined value over a wide range of rate of vapor or steam generation. This boiler or superheat control of recirculated gas flow will be subsequently described in detail.

Adapted to. be operative conjointly with the damper 112 is a tight shut off damper 120 disposed within the outlet duct 70 leading from the outlet of the recirculated asfa r As is well known in the art, the burners 42 have their firing rate controlledin response to steam or vapor demands. Such a demand may be indicated by pressure changes of' the generated steam or vapor and it may be also indicated by the rate of steam flow from the outlet of the secondarysuperheater. The rate of firing is subject to the combined or adjusted control of these influences and combined therewith there may be other influences such as the product of gas mass flow and temperature and vapor or steam temperature at another position in the steam fiowpath. 'In' any event, it is to be appreciated that th'e rate of firing of the furnace will change from full flow at top load to a minimum load as low as 25% ofjfull' load, and it is to be further appreciated that with such changes in rate of firing of the furnace the gas pressures and temperatures within the furnace will change over a considerable range. Concomitant with such with some types of burners. Control of air flow to the 75 changes in furnace gas temperature and flow resulting ffoni'changs in'the rate of firing of the burne'rs'are changes in the flow of recirculated gases to the unit. This is true Whether the recirculated gases are used to have a tempering effect upon the gasesor whether they are used directly to control superheat, or the final temperature of the steam or vapor issuing from-the unit. Under such variable gas flow conditions there is apt to be a condition in which the pressure of the unrecirculated heating" gases adjacent the inlet duct 70' or 70 and adjacent at the point of communication 72 or 72' with the lower part of the furnace will be greater than the pressure of the recirculated gases adjacent the fan 68 or 68. l Such a condition will cause a reverse flow of the heating gases through at least a part of the recirculated gas system, and substantial damage to-the fan is apt to result. This invention' with its control systems prevents such damage. The diiferential pressure switch 114 is a part of the system for closing the dampers 112, 120 and thereby preventing .such a reverse how of gases into the recirculated gas :system. This invention is further eflfective to prevent damage under such reverse flow conditions, by providing :for the entry of relatively low temperature air through :the bypass duct 86 or 86' into the outlet duct 70 or 70,

lleading from the recirculating gas fan. A part of :the devices for implementing this part of the method involves :an interlock between the dampers 120 and 88. operating in such a manner that when the damper 120 is completelyclosed the airdamper v88 (or 88) will be completely opened. ra-

- :Referring tothe control diagram illustrated in Fig. 3

i 1 of the drawings, lines 151 and 152. represent bus bars constituting parts of conductorsleading from an electric generator, and the lines 153 and 154 represent power-- lines for the pertinent control elements. Interposed rela tive to the "bus bars and the lines 153 and 154 are fuses 151 and 152. When the breaker auxiliary switch 158 closes/the fan motor line 155 is closed through the time delay relay 159 which may be situated at the boiler gauge board. This relay functions to delay the opening" of dampers 112 and 120 and the closing of damper 88 or 88 until the recirculating fan 68 or 68' is running at normal speed and with sufiicient discharge pressure to prevent reverse flow of hot gases from the furnace (or the heat exchange unit 90) into the recirculating fan discharge conduit 70 or 70. Pressure of the recirculating fan discharge against the closed damper 120 will open the relay I contacts 164 of the differential pressure switch 114.

When, after the fan 68 (or 68') has attained normal speed, the time delay relay contacts 160 close to cause two solenoid valves 165 and 166, in the line 156, to be energized to cause the damper 120 to open fully and the damper 88 (or 88) to close partly and the recirculating gas control-damper 112 to open to a position determined by the steam temperature control (Fig. 4). 'The recirculating gas flow and the resulting pressure drop across the recirculating gas control damper 112 will cause the contacts 168 and 164 of the difierential pressure switch relay 169 to remain open. The contacts 164 and 168 are preferably set to close at a pressure differential of 1" water between the inlet and outlet sides of the damper 112. If the fan 68 .(or.68) is operating atless than one half load and the pressure drop across damper 112 drops to 1" of water (indicating that a reverse fiow condition is approaching) the auxiliary relay 169, for the differential switch 114, is energized. This will immediately trip, or stop, the recirculating fan, sound an alarm, close damper 112 and open damper 88 (or 88). If the damper112 is opened wide by the Fig. 4 steam: temperature control then'pressure drop across the damper may drop to 1" or less, even though the recirculating fan is functioning properly and is operating at near'full load to prevent the tripping or disconnecting of the fan motor circuit. Under this condition the fan motor current'relay contacts 167 are open. These contime "167 'are preferably set to be closed at half rated motor load and below, andwill be opened when the motor; is operating abovehalf operatinglload. When the con-- tacts 167 are closed and the-relay contacts 168 for the pressure diiferential switchare also closed then the contacts 164 are opened-to deenergize solenoids 165,-166 andthereby close dampers 112 and 120 and open dampers 88'(or 88).

The recirculating gas fan 166 are energized when the recirculating gas fan 68' is in service, and their energization preferably throws the damper 112 (or 71) over to the superheatcontrol of Fig. 4. A

The Fig. 4 boiler control, or superheat control system, includes the fan 68 (or 68), its recirculated gas control damper 112, the tight shutofi damper 120, the cooling air damper 88, the control damper operator 170 (such as shown in U.S. Patent 2,536,184) having an op-, erative connection 172 to'the damper 112, and an operator 174 for the dampers 120 and 88' with interlocking and operating connections 176 and 178. Both of the damper'operators .170 and 174 are subject to control from a plurality of influences. One influence springs from the final steam temperature recorder-controller 180 (such as shown in U.S. Patent 2,155,986).- Fr'om'this controller pneumaticcontrol impulses are transmitted through the pilot valve 182 (such as shown in U.S..Pat

ent 2,054,464) and its outlet line 184 to relay 186 (such as shown in Gorrie Re. 21,804) and thence through the line 188 to a chamber of the totalizing relay 190 (such as shown in U.S. 2,098,913). The relay 190 receives, through the steam flow controller 192, pilot valve 194, and the line 196, control impulses representative of changes in steam flow. Y

The totalizingrelay 190 also receives other control impulses from the recorder controller 198. These impulsts are representative of changes in the product of gas mass flow and steam temperature. The outgoing im-. pulses from the recorder controller 198 audits pilot valve 200 proceed through line 202 to the totalizing relay 190. Outgoing impulses from the relay 190 result from the. combined effect of its three incoming impulses. These outgoing impulses proceed through the relay .204, the line through the lines 216 and 218 to the solenoid valves 220 and 222. Thence the outgoing impulses from the solenoid valves 220 and 222 proceed through the lines 224 and 226 to the damper control operators 174 and 170 respectively in order that the dampers 112 and 120. shall be closed and the damper 88' opened when the difference in pressure in the duct 66' and on opposite sides of the damper 112 drops below 1" of water.

2 Connected with the lines 210 and 226 is an auxiliary line 228 leading to the pressure switches 230 and 232 which are connected respectively to the signal lights 234 and 236. When the pressure in the line 228 indicates that the damper 112 is closed, the signal'lights 236 will show aired light and when the recirculating system is operated normally, the light 234 will show a green light.

In contact with therecirculated gas flow in the outlet duct 70 is a temperature responsive element 240, preferably connected by a line 242 to sound'an alarm and to operate an interlock when the temperature in the duct 70"reaches 50 F. above full load normal operating temperature. The damper interlock, controlled by variations in. line 242, closes the damper and opens the cooling air damper 88'. 1 I

In Fig. 5 the inlet ductwork 66' leading to theinlet damper solenoids, and

assaaas manually operable. There is also a manually operable dampei 302 disposed'inla lateral'duct 304 preferably leading from the forced .draft fan of the vapor generating and superheating unit tocommunication with the inlet duct 66', as shown. The outlet duct 70' for the fan 68' also has a series of manually operable dampers 306. Beyond these dampers the flow of recirculated gases through the ductwork 7i) and into the furnace located beyond the position 308 is controlled by a series of dampers 310. These dampers have a connecting link 312 So that they may be operated in unison by the link 314 which is pivoted at its upper end to a crank arm 316. The lower end of this link is connected to another crank arm 318 which is turned by the damper drive 320. This drive is preferably automatically actuatedas a result of control impulses through the line 322 and its branch 324. The dampers 310, or some of them, are operated by a crank arm 330 through its pivotal connection with a link 332. The latter is similarly actuated by a damper drive 334 having a crank arm 336 pivotally connected to the lower end of the link 332. The drive 334 is preferably automatically operated as a result of control impulses through the branch line 338 which is also. in communication with the line 322. This line leads from a three-way solenoid valve 340 which is connected or, wired to an interlock system through the connection 342 to close the dampers-310' when the fan motor becomes inoperative. 7

The drive 344 for the shutoff dampers 346 in the inlet ductwork 66' is automatically actuated as a result of impulses received through the line 348 which is connected to the three-way solenoid valve 350. This valve is wired by the connection 352 to the interlock system into which the three-way solenoid valve340 is wired. The damper drive 344 has its crank arm 354 connected to the crank arm 356 for the dampers 346, by links 358 and 359.

The recirculated gas flow differential pressure controller 360 has its opposite pressure chambers connected by the lines 362 and 364 to the ductwork 70 at positions 366 and 368 on the opposite sides of the series of dampers 310. This differential pressure controller with its associated control elements is intended to bev effective upon the dampers 310 and 346 in the manner previously described with reference to the dampers 112'and 120 of Fig. 4. i

Communicating with the inlet ductwork' 70 at a position between the series of dampers 306 and the series of dampers 310 is an air duct 370 leading from the forced draft fan. This duct has a damper 372 automatically operated by reason of its linkage 374 connection with a damper operator 376. This damper operator is actuated as a :result of the variations in air pressure in the lines 378 and 380 which are connected, respectively, to the threeway solenoid valves 382 and 384.

The operative impulses for the Fig. system so far described arise from a plurality of indicating sources which are indicated at 400, 402, 404 and 406. At 400 there is a recorder-controller, indicating the vapor temperature at the outlet of the secondary superheater. It is connected at 410 to a high temperature alarm and at 412 to a low temperature alarm. Its operating member 414-is efiective, through the pilot valve 416 to vary the pneumatic impulses in the line 418 leading to the selector relay 420. This relay has another of its chambers communicating witha line v422 leading fromrthe recordercontroller 402 of primary superheater temperature. This recorder-controller 402 has a connection at 424 to a high temperature alarm and another connection at 426 with a low temperature alarm. Its-operative member 428 is effective through the pilot valve 430 and its associated elements to transmit impulses through theline 422, representative of changes in the temperature of superheated steam at the outlet of the primary. superheater.

.. *Iheoutgoing'impulses from the selector relay 1420 are 444 of. the recorder-controller'404.

transmitted through the line 432 to a relay 434 which, in turn, transmits its outgoing impulses through the line.435 to the ratio relay 436.

The superheated steam flow recorder-controller 404 sends out impulses through the line 440 to the ratio relay 436. These impulses are sent through the agency of the pilot .valve 442 and its operative connection with the arm The instrument 406 registers the product of gas temperature in the furnace times the mass flow and changes in this product effect changes in the pneumatic loading in the line 446 through the agency of the pilot valve .448 and its .connection 450 with the operating arm 452 of the instrument 406.

Outgoing impulses from the ratio relay 436, resulting from the effect of one or more of its incoming influences proceeding through one or more of the lines 432, 440 and 446, pass through the line 454 to the relay 456. This relay'also receives impulses through the line.458 from the relay 460, the incoming impulses for this relay 460 being transmitted through the line 462 from the recirculated gas flow differential pressure controller 360 and its pilot valve 464. Control impulses from the relay 456 proceed through the line 458 and the selector valve 470 to and through the line 472 to the three-way solenoid valve 340.

During the normal operation of the Fig. 5 control system the relays 420, 434, 436 and 456 are so adjusted and connected that, as the temperature of the superheated vapor falls below a predetermined value, the control dampers 310 are opened to a greater extent .to admit more recirculated gas to the furnace. The relay 420 may be set in accordance with the desire of the plant operator to make changes in primary superheat temperature and/ or changes in secondary superheat temperature predominantly efiective in securing such a change in recirculated gas flow. Similarly, the ratio relay 436 may be set to change the effect on the gas flow dampers 310 bychanging the extent or timing of the incoming influences through the line 446, 440 and 435.

The relays 460 and 456 are so connected and/ or adjusted that the impulses received from the differential pressure controller 360 through the line 462 will be effective' through its operative connections to the dampers 310 and 346 to close the dampers in the event that the pressure differential between the positions 366 and 368 fallsbelow a predetermined value. This will take effect in spite of any control impulses of a normal nature passing through the line 454 to the relay 456.

Although the invention has been described with reference to the details of preferred embodiments, it is to be understood that the invention is not to be considered as limited to all of the details thereof. It is rather to be taken as of a scope commensurate with the scope of the subjoinedclaims.

What is claimed is: v

1. In a vapor generating and superheating unit, a furnace, means firing the furnace including fuel feeding means a forced draft fan, vapor generating tubes subject to the heat of the furnace and gases therefrom, a vapor superheater including superheater tubes connected to the outlets of the vapor generating tubes and subject to the heat of gases from the furnace, a recycled gas system including a fan and ductwork constructed and arranged to withdraw furnace gases after passing over superheater surface and controllably deliver the withdrawn gases for addition to the unrecycled gases passing through the zone of the vapor generating tubes to the superheater, said system having a fan outlet duct communicating with furnace gas flow at a position ahead of the superheater, means varying the amount of recycled gas delivery as the vapor generating rate and furnace firing rate change, said last named means including a control damper in said (11161? work, control means automatically closing the damper whenthe difference in the pressures in said ductwork and on the inlet and outlet sides of the. damper attains a predetermined value, a bypass having an inlet communicat 2. In a vapor generating and superheating unit, a furmeans and a forced draft fan, vapor generating tubes subject to the heat of the furnace and gases therefrom, a vapor superheater including superheatertubes subject to the heat of gases from the furnace, a recycled gas system includinga fan and ductwork constructed and arranged to withdraw furnace gases after passing over superheatersurface and controllably deliver the withdrawn gases for addition to the unrecycled gases passing to the superheater, said system having a fan outlet duct communicating with furnace gas flow at a position ahead of the superheater, means varying theamount of recycled gas delivery as the vapor generating rate and furnace firing rate change, said last named means including a control damper in said ductwork, control means automatically closing the damper when the difference in the pressures on the inlet and outlet sides of the damper attains a predetermined value, a bypass having an inlet communicating with the outlet of the forced draft fan and having an outlet communicating with the recycled gas fan outlet duct, a damper in the bypass, means automatically opening the bypass damper when the control damper is closed, and means responsive to the recycled gas fan load for rendering said control means inoperative when the load on the recycled gas fan is greater than a predetermined fraction of full load.

3. In a steam generating and superheating unit, a furnace, means firing the furnace, a steam superheater including steam conducting tubes subject to the flow of gases originating in the furnace, steam generating'tubes subject to the heat of the furnace, means conducting the generated steam to the superheater, a recycled gas sys tem having connected inlet ductwork communicating with 'nace, means firing the furnace including fuel feeding gas flow beyond the superheater and having connected outlet ductwork communicating with a gas flow zone ahead of the superheater, means varying the flow and pressure of gases leaving the furnace and flowing through said zone, recycled gas flow regulator means associated with said ductwork and responsive to a differential of recycled gas pressures on opposite sides of the regulator for preventing gas flow from said zone into the fan when the pressure differential drops below a predetermined value, means responsive to indications of load on the fan for rendering the differential regulator control means inoperative at fan loads above one half full load.

4. In a steam generating and superheating unit, a furnace, means firing the furnace, a steam superheater including steam conducting tubes subject to the flow of gases originating'inthe furnace, steam generating tubes subject to the heat of the furnace, means conducting the generated steam to the superheater, a recycled gas system having connected inlet ductwork communicating with gas flow beyond the superheater and having connected outlet ductwork communicating with unrecycled gas in a zone ahead of the superheater, means varying the flow and pressure of gases leaving the furnace and flowing through said zone, recycled gas flow regulator means associated with said ductwork and operable from a gas pressure differential on opposite sides of the regulator and in the outlet ductwork for closing the regulator and thereby preventing gas flow from said zone into the fan when the pressure differential drops below a predetermined value, means responsive to indications of load on the fan for rendering the differential regulator control means inoperative at fan loads above a predetermined fraction of full fan load.

5. In a steam generating and superheating unit, a furnace, means firing the furnace, a steam superheater in- 10 eluding steamlconducting tubes subject to the flow of high temperature gases originating in the furnace, steam generating tubes subject to the heat of the furnace, means conducting the generated steam to the superheater, a recycled gas system having a fan and connected inlet ductwork communicating with gas flow beyond the superheater and having connected outlet ductwork communicating with a gas flow zone ahead of the superheater, an electric motor for operating said fan, means varying the flow and pressure of gases leaving the furnace and flowing through said zone, recycled gas flow regulator means associated with said ductwork and operable from a differential of recycled gas pressures in said ductwork and on opposite sides of the regulator for closing the regulator and thereby preventing gas flow from said zone into the fan, means responsive to indications of load on the fan motor, for rendering the differential regulator control means inoperative at fan loads above a predetermined fraction of full fan load.

6. In a steam generating and superheating unit, a

furnace, means firing the furnace, said means including a forced draft fan, steam generating tubes subject to heat of the furnace gases, a steam superheater including superheater tubes subject to heat of the gases at a position beyond the furnace, means connecting the outlets of the generating tubes to the inlets of the superheater tubes, a recycling gas system includinga fan and ductwork having an inlet communicating with gas flow beyond the superheater and having an outlet communicating with the flow of unrecycled furnace gases in a zone ahead of the superheater, a gas flow regulator in said ductwork for varying the flow of recycled gas through said system and into said zone to control gas flow and gas temperature through said zone, pressure differential control means responsive to the difference between gas'pressures on opposite sides of the regulatorfor closing the regulator when the pressure differential falls below a predetermined value, and means whereby said outlet is. automatically supplied with air from the forced draft fan when the regulator becomes closed.

7. In a steam generating and superheating unit, a furnace, means firing the furnace, said firing means including a forced draft fan, steam generating tubes subject to heat of the furnace, a steam superheater including superheater tubes subject to heat of the gases at a position beyond the furnace, means connecting the outlets of the generating tubes to the inlets of the superheater tubes, a recycling. gas system including a fan and ductwork having an inlet communicating with gas flow beyond the superheater and having an outlet communicating with unrecycled furnace gases in a zone ahead of the superheater, a gas flow regulator in said ductwork for varying the flow of recycled gas through said system and into said zone to control gas flow and gas temperature through said zone, pressure differential control means responsive to the difference between gas pressures on opposite sides of the regulator for closing the regulator when the pressure difference falls below a predeterminedvalue, a bypass duct leading from the outlet of the forced draft fan to said ductwork, a damper in the bypass duct, and means opening the damper when the regulator closes. a i

8. In a vapor generating and superheating unit, a furnace, means firing the furnace including fuel feeding means and a forced draft fan, vapor generating tubes subject to the heat of the furnace and gases therefrom, a vapor superheater including superheater tubes subject to the heat of gases from the furnace, means connecting the outlets of the generating tubes to the inlets of the superheater tubes, a recycled gas system including a fan and ductwork constructed and arranged to withdraw furnace gases after passing over superheater surface and controllably deliver the withdrawn gases for addition to the unrecycled gases passing to the superheater, said system having a fan outlet duct communieating with furnace gas flow at, a position ahead of the superheater, means varying the amount of recycled gas delivery as the vapor generating rate and furnace firing rate change, said :last named means including a control damper in said ductwork, controlv means automatically closing the damper when the difference in gas attributes in said ductwork and on the inlet and outlet sides of the damper attains a predetermined value, a bypass having an inlet communicating with the outlet of the forced draft fan and having an outlet communicating with the recycled gas fan outlet duct, a damper in the bypass, and means automatically opening the bypass damper when the control damper is closed.

9. In a high pressure steam generating and superheating unit having a steam generating section and a superheater subject to the heat of high temperature gases from a furnace and having a fan operated gas recirculating system. returning gases from a position beyond the superheater for juncture with the unrecirculated gases at a position ahead of the superheater, means directly varying the rate of firing the furnace in accordance with steam demand, control means varying the amount of recirculated gas flow as the rate of firing varies, said control means including a damper normally in the flow of recirculated gases, pressure differential control means responsive to a predetermined pressure differential between a recirculated gas flow position upstream of the damper and a recirculated gas flow position downstream of the damper to automatically entirely close the damper when one or both of the variation of firing rate and the variation of recirculated gas flow results in said predetermined pressure differential, and means rendering said pressure differential control meansinoperative during the upper part of the load range of the recirculated gas fan.

10. In a steam generating and superheating unit having a furnace variably fired in accordance with varying steam demand, steam generating tubes subject to the heat of the, furnace gases, a superheater subject to the heat of gases from the furnace, for superheatingthe generated steam, and a recirculated gas system includinga fan and associated ductwork for withdrawing gases from a position beyond the superheater and variably introducing the withdrawn gases into the furnace at a position ahead of the superheater, said system having a damper between the fan and the furnace, means responsive to a predetermined difference between the gas pressures on opposite sides of the damper to close the damper when that pressure difference approaches a value at which flow of unrecirculated gases into the recirculated gas system would occur, and means for simultaneously introducing cooling air into the outlet portion of the recirculated gas system at a position between the fan and the furnace.

11. In a steam generating and superheating unit having a furnace variably fired in accordance with varying steam demand, a forced draft fan associated with the firing means, steam generating tubes subject to the heat of the furnace gases, a superheater subject to the heat of gases from the furnace for superheating the generated steam, a recirculated gas system including a fan and associated ductwork for withdrawing gases from a position beyond the superheater and variably introducing the withdrawn gases into the furnace at a position ahead" ofthe super- I2 heater, 'a damper in said ductwork between the fan and the furnace gases, control means detecting the pressure differential on opposite sides of the damper, means utilizing'that pressure differential to close the damper when that pressure differential approaches a value at which flow of unrecirculated gases from the furnace into the recirculated gas system would occur, and means simultaneously introducing cooling air from the forced draft fan into the outlet portion of the recirculated gas system at a position between said damper and the furnace.

12. In a vapor generating and superheating unit, walls including vapor generating tubes defining a furnace, means firing the furnace including a forced draft fan, a superheater subject to the heat of the furnace gases and connected for series flow fluid from said vapor generating tubes, a recirculated gas system including a gas fan, a gas fan inlet duct for withdrawing heating gases communicating with the flow of heating gases at a position downstream gas-wise of said superheater, a gas fan outlet duct for introducing the withdrawn gases into the furnace at a position upstream gas-wise of said superheater, a damper in said outlet duct, and differential pressure damper control means rendered effective while the gas fan is in operation to close said damper and to inactivate said gas fan in response to a decrease in the gas pressure differential between the inflow and outflow sides of said damper to a predetermined value at which reversal of gas flow from the furnace to the gas fan outlet is imminent and at which the furnace gas pressure is approaching but is less than the gas fan static pressure.

13. In a fluid heating unit, means defining a furnace, means firing the furnace including a forced draft fan, a fluid heater subject to the heat of the furnace gases, a recirculated gas system including a gas fan, a gas fan inlet duct for withdrawing heating gases communicating with the .flow of heating gases at a position downstream gas-wise of said fluid heater, a gas fan outlet duct for introducing the withdrawn gases into the furnace at a position upstream gas-wise of said fluid heater, a damper in one of said ducts for varying the flow of recirculated gases through said gas fan, and differential pressure damper control means rendered effective while the gas fan is in operation to initiate closing of said damper in response to a decrease. in the gas pressure differential between the inflow and outflow sides of said damper to a predetermined value at which reversal of gas flow from thefurnace to the gas fan outlet is imminent and at which the furnace gas pressure is approaching but is less than the fan static pressure.

References Cited in the file of this patent UNITED STATES PATENTS 1,873,656 Meachem Aug. 23, 193.2 2,283,745. Lines May 19, 19.42 2,659,324 7 Peple Nov. 17, 19,53 FOREIGN PATENTS 503,778 Belgium June 30; 1951 514,687 Belgium Oct. 31, 1952 523,870 Great Britain July 24, 1940 523,871 Great Britain July 24, 1940 371,985 Italy June 12, 1939 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0 2332 288 April l2 1960 Arlington Ro Harman et all It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8 line 58 after "means" insert and -,t' column 9 line 9 after "tubes" insert connected to the outlets of the vapor generating tubes and Signed and sealed this 1st day of November 1960.

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

